1. Field of the Invention
The present invention generally relates to a blood flow measuring apparatus configured to accurately measure a blood supply state without being influenced by an oxygen saturation concentration of the blood, and to a brain activity measuring apparatus using the blood flow measuring apparatus.
2. Description of the Related Art
As apparatuses to measure a blood flow, for example, there have been brain activity measuring apparatuses, which are used by wearing a probe that forms an optical waveguide on a head, measuring a blood flow of a brain, and displaying an image of an activity state of the brain on a monitor (Patent Document 1).
As another brain activity measuring apparatus, there has been an apparatus including an optical source to irradiate a living body with light, a light measuring unit including an optical transceiver which detects light with plural wavelengths emitted from the living body, a change measuring unit to measure a change over time of a specific component included in the blood according to a change in an amount of the transmitted light with the plural wavelengths, and a blood flow calculating unit to calculate a blood flow according to the change over time of the specific component and a proportion of the specific component in the blood (for example, see Patent Document 2). The apparatuses disclosed in Patent Documents 1 and 2 are also called optical topography apparatuses, whereby plural light emitting parts and light receiving parts are mounted on a head and an amount of transmitted light which has propagated inside a brain is detected by using near-infrared spectroscopy, so as to map an activity state of a brain function.
As blood flow measuring apparatuses to measure a blood flow of parts other than a brain, there has been an apparatus to measure a presence or absence of a blood clot. In this apparatus, the blood layer is irradiated with light and an amount of light which has transmitted through the blood layer is measured to detect the blood clot (for example, see Patent Document 3).
By the methods to measure a blood flow by using a light emitting part and a light receiving part which form an optical waveguide, such as those employed by the apparatuses disclosed in Patent Documents 1 to 3, a change in amount of light transmitted through blood has been measured. However, an amount or density (hematocrit) of red blood cells, which varies in accordance with a brain activity, has not been measured. It is known that hemoglobin (Hb) included in red blood cells has a property to absorb and scatteringly reflect light, and its optical characteristics are influenced by a Hb density, oxygen saturation, and an optical path length in the blood. Therefore, by the method of measuring a blood flow by using the light measuring unit as described above, a measurement result is changed depending on two conditions: namely, hemoglobin included in red blood cells and oxygen saturation (an oxygen amount carried by the red blood cells).
Therefore, when oxygen saturation of blood is constant, a blood flow can be accurately measured based on an amount of transmitted light that depends on an amount or density (hematocrit) of red blood cells in the blood. However, when oxygen consumption is increased or decreased by activities of a brain and muscles, the oxygen saturation is changed by an oxygen partial pressure (PaO2), which changes an optical absorption factor. As a result, there is a possibility in that a change of the amount of transmitted light caused by the change of oxygen saturation is also measured as a change of the blood flow.
[Patent Document 1] Japanese Patent Application Publication No. 2003-149137
[Patent Document 2] Japanese Patent Application Publication No. 2003-144401
[Patent Document 3] Japanese Patent Application Publication No. 2002-345787
In the case of measuring a blood flow in a blood vessel for supplying blood to a brain or muscles by using the measuring apparatuses disclosed in Patent Documents 1 through 3, it has been difficult to accurately measure an activity state of the brain and muscles since the oxygen saturation changes depending on the oxygen partial pressure in the blood, which changes when the brain or muscles are highly active.
When the activity of the brain becomes greater, oxygen consumption of the brain increases. Therefore, multiple capillaries supply blood to the brain. Thus, a blood flow of a predetermined region, where plural capillaries are present, is measured depending on the size of a sensor (diameter of a probe which forms an optical waveguide). However, in the case where blood flows with different oxygen saturations in the plural capillaries, the conventional blood flow measuring apparatus and brain activity measuring apparatus have also detected a change in an amount of transmitted light that is caused by the change of the oxygen saturation. Therefore, it has been difficult to accurately measure an activity state of the brain.
In the case of measuring a blood flow in a blood vessel of other than a brain, it has been difficult to accurately measure the blood flow when the oxygen saturation of blood is not constant. It is because the amount of transmitted light changes depending on factors of both the density (hematocrit) or amount of red blood cells and the oxygen saturation.
In view of the above-described circumstances, it is an object of at least one embodiment of the present invention to provide a blood flow measuring apparatus that solves the above problems and a brain activity measuring apparatus using the blood measuring apparatus.
To solve the above-described problems, the present invention provides the following measures.